2 wire vs 3 wire thermostat

I have Oil fired hot water baseboard heat, with 2 zones. I have wanted to install a programmable thermostat, but I cannot find one that works with 3 wire aquastat.
What is the difference between 3 wire & 2wire thermostats?
Is there a way for me to adapt a prog therm. for use in 3 wire system?

It has been over 30 years since I worked on wet heat, but I do not ever remember needing a three wire thermostat for just heating purposes. Usually the aquastat has two terminals and the thermostat just switches them on or off.